This exact same argument could be used on the theoretical pacemaker in another question. But he believes the pacemaker should free software. His response there could have been tongue-in-cheek, but I can't quite tell.

Isn't he using these two situations to show how he can advocate a reasonable common ground for free software advocates, but he personally holds himself to a higher standard as he tries to be the beacon for Free Software, and secondarily, that to replace non-free software, sometimes you have to closely blackbox it:

"The only way I could justify this is if I began developing a free replacement for that very program. It is ok to use a nonfree program for the purpose of developing its free replacement." - RMS answering 22.